Homoepitaxial tunnel barriers with functionalized graphene-on-graphene and methods of making

ABSTRACT

This disclosure describes a method of making a tunnel barrier-based electronic device, in which the tunnel barrier and transport channel are made of the same material—graphene. A homoepitaxial tunnel barrier/transport device is created using a monolayer chemically modified graphene sheet as a tunnel barrier on another monolayer graphene sheet. This device displays enhanced spintronic properties over heteroepitaxial devices and is the first to use graphene as both the tunnel barrier and channel.

This application claims priority to and the benefits of U.S. PatentApplication No. 61/980,448 filed on Apr. 16, 2014, the entirety of whichis herein incorporated by reference.

BACKGROUND

The quantum phenomenon of tunneling enables novel charge-based deviceswith ultra-low power consumption, and is key to the emerging field ofspintronics.

Tunnel devices typically require mating dissimilar materials andmaintaining monolayer level control of thickness, raising issues thatseverely complicate fabrication and compromise performance. The recentdiscoveries of intrinsically 2-dimensional materials such as grapheneand h-BN have created new perspectives on tunnel barriers. Their strongin-plane bonding promotes self-healing of pinholes and a well-definedlayer thickness, important because the tunnel current dependsexponentially upon the barrier thickness.

There has been keen interest in utilizing graphene, a two-dimensional(2D) honeycomb lattice of carbon atoms, as a high mobility transportchannel. Its linear band dispersion, ambipolar conduction, andremarkable in-plane electronic transport properties have stimulateddevelopment of RF transistors and wafer-scale fabrication of graphenecircuits. Graphene also exhibits exceptional in-plane spin transportcharacteristics, including long spin diffusion lengths due to its lowspin-orbit interaction, which has stimulated ideas for novel spindevices.

The highest values for spin diffusion lengths and spin lifetimes havebeen measured using mechanically exfoliated graphene, which, although itpossesses extraordinary electrical properties, is not amenable fordevice scalability, as devices must be fabricated on individual,randomly placed and sized flakes. Moreover, spin injection into graphenefrom a ferromagnetic metal contact typically requires the use of anoxide tunnel barrier such as Al₂O₃ or MgO to accommodate the largeconductivity mismatch. These materials do not wet the graphene surface,making it very difficult to control the thickness and uniformity of thetunnel barrier.

In addition, the mobility of graphene is significantly degraded bycoupling to phonons or charged impurities/defects in an adjacent oxide.Consequently, significant effort has focused on exploiting other carbonthin films and 2D materials such as h-BN or MoS₂ as a substrate, gatedielectric, or tunnel barrier for graphene devices. This improvesoperating characteristics, but significantly complicates thefabrication, and often relies upon sequential mechanical exfoliation toproduce a few device structures.

Although single layer graphene itself has been shown to function as atunnel barrier in a heterostructure, it does not effectively serve as atunnel barrier on another layer of graphene because there is electricalinteraction between the two layers, regardless of the stackingorientation, except in a large magnetic field.

One can markedly alter graphene's physical properties with chemicalfunctionalization by fluorination or hydrogenation. Fluorinated grapheneis an excellent in-plane insulator, and no electrical communication isobserved between adjacent layers of fluorographene and graphene,allowing for its use as a tunnel barrier in an all-graphenetunnel-transport homoepitaxial structure.

Only two other methods have been devised for making tunnel barriers ongraphene. First, a method of high-energy electron-beam lithographicdecomposition of vaporized carbon can produce amorphous carbon layers onthe surface of the graphene channel and can act as a tunnel barrier.Although this method produces tunnel barriers, the high-energy electronbeam adds charged impurities to the substrate, affecting the transportproperties of the graphene channel, and it can induce physical damage tothe graphene by driving off individual carbon atoms from the lattice. Asecond alternative method involves the chemical vapor deposition growthof thin layer hexagonal-BN, which is then transferred to the graphenetransfer. However, this process does not produce exceptional results andis not homoepitaxial, requiring the growth and transfer of twocompletely different materials with vastly different growth mechanismsand properties. Thus, it is also unsuitable for industrial scaling.

BRIEF SUMMARY OF THE INVENTION

This disclosure describes a process to fabricate a completely new kindof tunnel barrier-based electronic device, in which the tunnel barrierand transport channel are made of the same material—graphene. Anever-before-seen homoepitaxial tunnel barrier/transport structure iscreated using a monolayer chemically modified graphene sheet as a tunnelbarrier on another monolayer graphene sheet. The new type of devicedisplays enhanced spintronic properties over heteroepitaxial devices andis the first to use graphene as both the tunnel barrier and channel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, and 1C illustrate images and electrical characterizationbefore and after device fluorination. FIG. 1A illustrates an opticalimage of a device with fluorination only under the Py contacts. Thescale bar is 20 μm. FIG. 1B illustrates an optical image of a devicefluorinated everywhere. The contrast of the upper layer of graphenechanges, becoming transparent, as it is fluorinated. The scale bar is 20μm. FIG. 1C illustrates an IV curve of a 2T graphene device beforefluorination. FIG. 1D illustrates an IV curve of the same device afterfluorination, showing that the graphene is now an insulator.

FIGS. 2A, 2B, and 2C are demonstrations of tunneling behavior. FIG. 2Aillustrates Dirac curves taken using the SiO₂/Si substrate as a backgate. One line illustrates between contacts 2 and 3 (only on thefluorographene). Another line illustrates between contacts 1 and 4 (onlyon the graphene channel). The fluorographene shows no Dirac point,indicating that it is fully insulating and electrically uncoupled fromthe graphene that is underneath it. FIG. 2B illustrates current-voltagecurves for a typical device. Taken between contacts 1 and 2 or 1 and 3(includes the tunnel barrier), the curves are non-Ohmic. Taken betweencontacts 1 and 4 (graphene channel only) the curve is linear. The insetfurther highlights this by showing dV/dI vs. V when the tunnel barrieris included in the circuit. FIG. 2C illustrates zero bias resistance vs.temperature for the Py contacts showing a weak temperature dependence(non-metallic behavior) that is a hallmark of a good tunnel barrier.

FIGS. 3A, 3B, 3C, and 3D illustrate spin transport data. FIG. 3Aillustrates non-local spin valve (NLSV) measurement taken at 10 K. Thearrows below the curve indicate the direction of the field sweep. Thearrows above the curve indicate the ferromagnetic contact magnetizationdirections. A small constant background of ˜400 Ohms was subtracted fromthe data. FIG. 3B illustrates 4T Hanle (top and right axes) vs. 2T Hanleand witness sample (bottom and left axes). All data taken at 10 K. Thedotted lines show fits to the appropriate models. Here, τ_(s) for 2T/4Twas 96 ps/205 ps. FIG. 3C illustrates bias dependence of 2T (triangle)and 4T (square) Hanle signal amplitude. FIG. 3D illustrates biasdependence of the NLSV plateau ΔR_(NL) and the spin polarizationefficiency, showing evidence of spin-filtered tunneling.

FIG. 4 illustrates gate voltage dependence of the 4T spin lifetime at 10K. The bias current was −10 μA.

DETAILED DESCRIPTION

This disclosure describes a process to fabricate a completely new kindof tunnel barrier-based electronic device, in which the tunnel barrierand transport channel are made of the same material, graphene. Anever-before-seen homoepitaxial tunnel barrier/transport structure iscreated using a monolayer chemically modified graphene sheet as a tunnelbarrier on another monolayer graphene sheet. The new type of devicedisplays enhanced spintronic properties over heteroepitaxial devices andis the first to use graphene as both the tunnel barrier and channel.

The quantum phenomenon of tunneling enables novel charge-based deviceswith ultra-low power consumption, and is key to the emerging field ofspintronics. Tunnel devices typically require mating dissimilarmaterials and maintaining monolayer level control of thickness, raisingissues that severely complicate fabrication and compromise performance.The recent discoveries of intrinsically 2-dimensional materials such asgraphene and h-BN have created new perspectives on tunnel barriers.Their strong in-plane bonding promotes self-healing of pinholes and awell-defined layer thickness, important because the tunnel currentdepends exponentially upon the barrier thickness.

There has been keen interest in utilizing graphene, a two-dimensional(2D) honeycomb lattice of carbon atoms, as a high mobility transportchannel. Its linear band dispersion, ambipolar conduction, andremarkable in-plane electronic transport properties have stimulateddevelopment of RF transistors and wafer-scale fabrication of graphenecircuits. Graphene also exhibits exceptional in-plane spin transportcharacteristics, including long spin diffusion lengths due to its lowspin-orbit interaction, which has stimulated ideas for novel spindevices. The highest values for spin diffusion lengths and spinlifetimes have been measured using mechanically exfoliated graphene,which, although it possesses extraordinary electrical properties, is notamenable for device scalability, as devices must be fabricated onindividual, randomly placed and sized flakes. Moreover, spin injectioninto graphene from a ferromagnetic metal contact typically requires theuse of an oxide tunnel barrier such as Al₂O₃ or MgO to accommodate thelarge conductivity mismatch. These materials do not wet the graphenesurface, making it very difficult to control the thickness anduniformity of the tunnel barrier. In addition, the mobility of grapheneis significantly degraded by coupling to phonons or chargedimpurities/defects in an adjacent oxide. Consequently, significanteffort has focused on exploiting other carbon thin films and 2Dmaterials such as h-BN or MoS₂ as a substrate, gate dielectric, ortunnel barrier for graphene devices. This improves operatingcharacteristics, but significantly complicates the fabrication, andoften relies upon sequential mechanical exfoliation to produce a fewdevice structures.

Although single layer graphene itself has been shown to function as atunnel barrier in a heterostructure, it does not effectively serve as atunnel barrier on another layer of graphene because there is electricalinteraction between the two layers, regardless of the stackingorientation, except in a large magnetic field. One can markedly altergraphene's physical properties with chemical functionalization byfluorination or hydrogenation. Fluorinated graphene is an excellentin-plane insulator, and no electrical communication is observed betweenadjacent layers of fluorographene and graphene, suggesting its use as atunnel barrier in an all-graphene tunnel-transport homoepitaxialstructure.

Here, described is a method of fabrication and demonstration of theoperation of the world's first homoepitaxial graphene-on-graphene tunnelbarrier/transport structure.

Demonstrated is the increased performance of our new structure byfabricating spintronic devices where a monolayer of chemicallyfunctionalized graphene acts as a tunnel barrier on a monolayer ofnon-functionalized graphene, and demonstrate electrical spin injection,lateral transport, and detection by 4-terminal non-local spin valve andHanle effect measurements. We find the highest spin efficiency valuesyet measured for graphene, and present evidence for the theoreticallypredicted enhancement of tunnel spin polarization.

Example 1 Formation of the Homoepitaxial Graphene TunnelBarrier/Transport Channel Device

Graphene was grown by chemical vapor deposition (CVD) via decompositionof methane in small Cu foil enclosures. This method produces monolayergraphene films with grain sizes on the order of hundreds of micronscontaining minimal defects. After growth, the graphene is removed fromthe Cu growth substrate by etching the copper in acid solution, and thenit is mechanically transferred onto a SiO₂/Si substrate.

Care is taken to eliminate the exposure of graphene to standard opticalphotoresists, which can leave significant residues on graphene. Instead,a PMMA-based process is used, which produces fewer residues. The firstlayer of graphene is spin-coated with a thin layer of PMMA followed byShipley S1818 photoresist. Using photolithography, a mesa pattern isdefined in the photoresist and O₂ plasma is used to etch through thePMMA and unwanted graphene. The sample is rinsed in acetone andisopropyl alcohol (IPA) to remove the etch mask. Ohmic referencecontacts and bond pads are then defined using a MMA/PMMA mask withfeatures defined using high-current (˜7 nA) electron-beam lithographywriting. Ti/Au is deposited using electron beam deposition and lift-offin acetone.

A second layer of graphene that will act as the tunnel barrier is thendeposited on top of these devices using the same methods as above. Asecond mesa etch, similar to the first, is performed. Electron-beamlithography using a MMA/PMMA resist is then used to define trenches fordeposition of ferromagnetic contacts by electron beam deposition andlift-off. The graphene in these trenches is fluorinated by placing thesample in XeF₂ gas until the resistance of a concurrently fluorinated2-terminal graphene device reaches approximately 50 GΩ, indicating thatthe upper graphene layer is fully insulating. This is shown in FIG. 1.NiFe/Au is then deposited by electron beam evaporation and lift-off isperformed in acetone.

Just prior to placing the devices in a cryostat for measurement, a finalfluorination is performed to fluorinate the remaining upper layer ofgraphene to prevent any edge state conduction. Again, IV characteristicsof a concurrently fluorinated 2-terminal graphene devices are measuredand the observation of high resistance ensures that the fluorinationprocess has succeeded. After this fluorination, the upper layer ofgraphene (now fluorographene) changes optical contrast, becoming visiblyalmost transparent. This contrast change is further evidence that thefluorination was successful.

Example 2 Demonstration of Tunneling Behavior

FIG. 1B shows an image of the device structure. It consists of twoferromagnetic permalloy/fluorinated graphene tunnel contacts (contacts 2and 3) placed between two Au/Ti contacts (1 and 4). The Au/Ti contactsshow Ohmic behavior, as expected. The Conductance vs. Back Gate Voltage,measured between the two Ohmic Au/Ti contacts (FIG. 2A), shows the Diracpoint of the bottom graphene channel at ˜80V, indicating a high electronconcentration. The transistor characteristics measured between the twopermalloy (Py, Ni₈₀Fe₂₀) contacts that only contact the top fluorinatedgraphene film (FIG. 2A), shows no modulation or Dirac point. Thisconfirms that the graphene layers are indeed not communicatingelectrically, as expected after fluorination. The conductance of thedevice between these two Py contacts is orders of magnitude less thanthe conductance of the fluorinated graphene film, indicating that all ofthe electrical transport measured is due to tunneling through thefluorinated graphene and into the underlying graphene transport channel.

FIG. 2B shows IV curves taken between the Py and the Ohmic Au/Ticontacts. These curves exhibit markedly non-Ohmic behavior, furtheremphasized in the inset of FIG. 2B with a graph of the differentialconductance vs. voltage, and provide additional support that thefluorinated graphene is acting as a tunnel barrier. The temperaturedependence of the zero bias resistance (FIG. 2C) is weak andinsulator-like in character, changing by a factor less than 1.7 for bothPy contacts. Non-Ohmic IV curves and a weakly temperature dependent zerobias resistance has been shown to be firm confirmation of tunnelingbehavior in the contacts.

Example 3 Operation of the Device as a Spin Valve

In non-local spin valve (NLSV) measurements, a bias current is appliedbetween one of the FM contacts and the nearest Ohmic reference contact,and a spin-polarized charge current is injected from the FM across thefluorinated graphene tunnel barrier and into the graphene transportchannel. Spin simultaneously diffuses in all directions, creating a purespin current on one side, and the corresponding spin accumulationresults in a spin-splitting of the chemical potential. This ismanifested as a voltage on the second FM contact, which is outside ofthe charge current path and referred to as the non-local detector. Anin-plane magnetic field is used to control the relative orientation ofthe magnetizations of the FM injector and detector contacts. When themagnetizations are parallel, the voltage measured will be smaller thanwhen they are antiparallel. Sweeping the magnetic field causes thecontact magnetizations to reverse in-plane at their respective coercivefields and produce a measurable voltage peak.

In order to observe this effect, we fabricate the Py contacts with twodifferent widths (0.5 μm and 3 μm) to exploit magnetic shape anisotropyso that the coercivities of the ferromagnetic contacts are different.This NLSV behavior is clearly observed in FIG. 3A, where distinct stepsin the non-local resistance (the measured voltage divided by the biascurrent) appear at the coercive fields of the wide and narrow FMcontacts, producing plateaus of higher resistance when the FM contactmagnetizations are antiparallel. This demonstrates successful spininjection and detection at the FM/fluorinated graphene tunnel contacts,and lateral spin transport in the graphene channel.

The spin lifetime corresponding to this pure spin current isquantitatively determined using the Hanle effect, in which a magneticfield B_(z) applied along the surface normal causes the spins in thegraphene transport channel to precess at the Larmor frequency,ω_(L)=gμ_(B)B_(z)/

, and dephase. Here g is the Lande g-factor (g˜2 for graphene), μ_(B) isthe Bohr magneton, and

is Planck's constant. As the magnetic field increases, the net spinpolarization and corresponding spin voltage decreases to zero with acharacteristic pseudo-Lorentzian line shape. FIG. 3B shows Hanle spinprecession curves for both non-local and local contact geometries for atypical device used in this study in comparison to a witness sampledevice where the top graphene layer was not fluorinated. We note that noNLSV signal or Hanle effect is apparent in the witness sample,demonstrating that the fluorinated graphene tunnel barrier is necessaryto achieve spin injection.

We measure Hanle spin precession in two different electricalconfigurations. The spin lifetime of the pure spin current is measuredin the NLSV or 4T configuration, where the full-width-half-max of themeasured change in voltage is directly proportional to the steady-statespin polarization at the detector, given by

$\begin{matrix}{{S\left( {x_{1},x_{2},B_{z}} \right)} = {S_{0}{\int_{0}^{\infty}{\frac{1}{\sqrt{4\;\pi\; D\; t}}\ {\mathbb{e}}^{{- {({x_{2} - x_{1} - {v_{d}t}})}^{2}}\text{/}4\; D\; t}{\cos\left( {\omega_{L}t} \right)}{\mathbb{e}}^{{- t}/\tau_{s}}{\mathbb{d}t}}}}} & (1)\end{matrix}$where spin is injected into the graphene at x₁ and t=0 and detected atx₂. S₀ is the spin injection rate, D is the electron diffusion constant,v_(d) is the electron drift velocity (=0 for diffusive transport), andτ_(s) is the spin lifetime. Secondly, the spin current can be injectedand the spin voltage detected with same Py contact in a 2-terminal (2T)configuration. Here, we measure the spin accumulation and lifetimedirectly under the Py contact, and the voltage ΔV_(2T)(B_(z)) decreaseswith B_(z) with a Lorentzian line shape given byΔV_(2T)(B_(z))=ΔV_(2T)(0)/[1+(ω_(L)t_(s))²]. In this way, fits to theHanle curves allow us to extract the spin lifetime (for the 2T and 4Tcase) and the spin diffusion constant (for the 4T case).

In FIG. 3B we see a strong Hanle signal from the 4T non-localmeasurement and the 2T measurement. The Hanle signal persists up to ˜200K. Average 4T spin lifetimes were ˜200 ps and average 2T spin lifetimeswere ˜100 ps. The spin diffusion length is given byL_(SD)=(Dt_(s))^(1/2) where D is the diffusion constant. We find anaverage L_(SD)˜1.5 μm, based on t_(s)˜200 ps and D˜0.01224 m²/s. Theobservation of both the non-local Hanle effect and the NLSV providesstrong evidence that the fluorinated graphene tunnel barrier indeedenables efficient spin injection, transport, and detection in thegraphene channel. Based on the magnitude of the NLSV signal (FIG. 3A)and the calculated spin diffusion length from the 4T Hanle measurements,we can determine the tunneling spin polarization, P, of thePy/fluorinated graphene contact using the formula:

$\begin{matrix}{{\Delta\; R_{N\; L}} = {\frac{P^{2}L_{S\; D}}{W\;\sigma}{\exp\left( \frac{- L}{L_{S\; D}} \right)}}} & (2)\end{matrix}$where σ is the measured conductivity of 1.29×10⁻⁴Ω⁻¹ for the deviceshown in FIG. 3A, L is the center to center contact spacing of 5.75 μm,L_(SD) is the spin diffusion length of 1.5 μm, W is the width of thegraphene channel of 5 μm, and ΔR_(NL)˜3.3Ω is the magnitude of the NLSVplateau for a bias current of −10 μA. From this, we find P-26%. FIG. 3Dsummarizes the bias dependence ΔR_(NL) and P. Both increasemonotonically with decreasing bias, typical of graphene NLSV devices. Wemeasure values of P up to ˜45% at low bias, which is at the upper limitof what can be expected for an intrinsic spin polarization of Ni₈₀Fe₂₀,which is 32%-48%. This value is also larger than the highest valuesmeasured to date (P=26-30%) in graphene NLSV devices with alumina or MgOtunnel barriers. This indicates that spin-filtering occurs at thePy/fluorinated graphene interface, consistent with theoreticalpredictions for spin transport across Ni/graphene lattice-matchedinterfaces, and provides further evidence for the efficacy of thefluorinated graphene tunnel barrier.

As a final demonstration of the effectiveness of our homoepitaxialstructure, we show gate modulation of the spin lifetime in FIG. 4. Whilemost early graphene spin experiments showed spin lifetimes that areconstant in gate voltage, other work shows that spin lifetimes areaffected by changes in carrier density. Studies that observe gatevoltage dependence have in common high contact resistance in the oxidetunnel barrier contacts, indicative of pinhole-free tunnel barriers thatprevent back diffusion into the FM contact and subsequent fast spinrelaxation. The discrepancy between these measurements is thus likelyrelated to differences in the quality of the tunnel barrier contacts. Inall of these cases, it would be difficult to measure the intrinsicproperties of the graphene itself. The single-atom thick fluorinatedgraphene tunnel barrier offers an elegant solution. Our experiments showa clear gate voltage dependent spin signal that follows the Dirac curve,just as predicted by theory.

Our structure demonstrates the first homoepitaxial tunnelbarrier/transport system in which the tunnel barrier and transportchannel are comprised of the same material, graphene. In previous art,the tunnel barrier and transport channel are very different materials,and such devices require mating dissimilar materials, raising issues ofheteroepitaxy, layer uniformity, interface stability and electronicdefect states that severely complicate fabrication and compromiseperformance.

Our new approach obviates these issues. Our approach does not rely upona second material “wetting” the graphene surface to obtain a uniform andcomplete tunnel barrier. Graphene, by definition, is uniform inthickness down to a single atom, has very few defects, does not easilyform vacancies, and does not intermix readily with other materials—theseare key characteristics for a tunnel barrier, in which the tunnelcurrent depends exponentially on the barrier thickness.

Our approach provides a simple and effective way to form a tunnelbarrier on graphene. The functionalized graphene tunnel barrier does notaffect the adjacent transport channel because it is comprised of thesame material, contrary to evaporated dielectric or oxidized metaltunnel barriers, which can structurally damage the graphene or addimpurity dopants. This is readily indicated by our high spinpolarization values, spin relaxation lengths on par with the highestquality graphene devices, and our ability to control the spin relaxationtime with the application of an electrostatic back gate.

Our complete tunnel barrier/transport channel structure also providesfor the thinnest of this type of structure ever made, allowing it to beused in applications where space is a premium. Furthermore, due to thethinness of the tunnel barrier, and the advantage that it allows fortrue electron tunneling, our structure has lower impedance and less lossthan other previously made designs, allowing its use in ultra low-powerelectronics architectures.

A majority of previous tunnel barrier devices using graphene as theconductive transport channel rely on deposited oxides or post depositionoxidized metals, usually both consisting of Al₂O₃ or MgO. The depositionis performed with three types of methods: 1) evaporative methods witheither thermal or electron beam evaporation to deposit an oxide. Theevaporated oxide or metal tends to ball up on the surface, causingcracks and pinholes that limit tunneling. 2) Sputter evaporation ofoxide or metal. It has been shown that the graphene transport channelcan be irreversibly damaged. 3) Atomic layer deposition of oxides.Successful deposition usually requires a chemical pretreatment of thegraphene film, which adds dopants that affect the transport properties.Moreover, oxide tunnel barriers are known to be very difficult to formon graphene since they exhibit de-wetting in the absence of priorchemical treatment of the graphene, and attempts to mitigate this tocreate a good surface for oxide growth may induce scatterers anddefects.

Many modifications and variations of the present invention are possiblein light of the above teachings. It is therefore to be understood thatthe claimed invention may be practiced otherwise than as specificallydescribed. Any reference to claim elements in the singular, e.g., usingthe articles “a,” “an,” “the,” or “said” is not construed as limitingthe element to the singular.

What is claimed is:
 1. A method of making a homoepitaxial tunnel barriertransport device with functionalized graphene-on-graphene, comprising:growing a first monolayer graphene film; transferring the firstmonolayer graphene film onto a SiO₂/Si substrate; spin-coating a layerof PMMA onto the first monolayer graphene film; applying a layer ofphotoresist onto the layer of PMMA; defining a mesa pattern usingphotolithography in the layer of photoresist; etching through the layerof PMMA and portions of the first monolayer graphene film with an oxygenplasma; defining ohmic reference contacts and bond pads using a MMA/PMMAmask; defining features using high-current electron-beam lithography;depositing Ti/Au using electron-beam deposition thereby forming apartial device; growing a second monolayer graphene film; transferringthe second monolayer graphene film onto the top of the partial device;spin-coating a second layer of PMMA onto the second monolayer graphenefilm; spin-coating a second layer of photoresist onto the second layerof PMMA; defining a second mesa pattern using photolithography in thesecond layer of photoresist; etching through the second layer of PMMAand portions of the second monolayer graphene film with an oxygenplasma; defining trenches for deposition of ferromagnetic contacts byelectron beam deposition and lift-off; fluorinating the second monolayergraphene film; depositing NiFe/Au by electron beam evaporation therebyforming a device; and performing a second fluorination to fluorinate thedevice thereby forming the homoepitaxial tunnel barrier transport devicewith functionalized graphene-on-graphene.
 2. The method of making ahomoepitaxial tunnel barrier transport device with functionalizedgraphene-on-graphene of claim 1, wherein the step of fluorinating thesecond monolayer graphene film further comprises the step of placing thepartial device in XeF₂ gas until the resistance of a concurrentlyfluorinated 2-terminal graphene device reaches about 50 GΩ.
 3. Themethod of making a homoepitaxial tunnel barrier transport device withfunctionalized graphene-on-graphene of claim 2, wherein the step ofperforming a second fluorination to fluorinate the device furthercomprises the step of preventing any edge state conduction.
 4. Themethod of making a homoepitaxial tunnel barrier transport device withfunctionalized graphene-on-graphene of claim 1, further comprising thestep of utilizing the tunneling behavior.
 5. The method of making ahomoepitaxial tunnel barrier transport device with functionalizedgraphene-on-graphene of claim 2, further comprising the step ofoperating the homoepitaxial tunnel barrier transport device withfunctionalized graphene-on-graphene as a spin valve.
 6. A method ofmaking a homoepitaxial tunnel barrier transport device withfunctionalized graphene-on-graphene, comprising: growing a firstmonolayer graphene film; transferring the first monolayer graphene filmonto a SiO₂/Si substrate; preparing a partial device; growing a secondmonolayer graphene film; transferring the second monolayer graphene filmonto the top of the partial device; preparing a second partial device;fluorinating the second monolayer graphene film; and forming thehomoepitaxial tunnel barrier transport device with functionalizedgraphene-on-graphene.
 7. The method of making a homoepitaxial tunnelbarrier transport device with functionalized graphene-on-graphene ofclaim 6, further comprising: performing a second fluorination tofluorinate the homoepitaxial tunnel barrier transport device withfunctionalized graphene-on-graphene.
 8. The method of making ahomoepitaxial tunnel barrier transport device with functionalizedgraphene-on-graphene of claim 7, further comprising the step ofutilizing the tunneling behavior.
 9. The method of making ahomoepitaxial tunnel barrier transport device with functionalizedgraphene-on-graphene of claim 7, further comprising the step ofoperating the homoepitaxial tunnel barrier transport device withfunctionalized graphene-on-graphene as a spin valve.
 10. The method ofmaking a homoepitaxial tunnel barrier transport device withfunctionalized graphene-on-graphene of claim 7 further comprising thestep of preventing any edge state conduction.
 11. A homoepitaxial tunnelbarrier transport device with functionalized graphene-on-graphene,comprising: a substrate; a monolayer graphene film; and a chemicallymodified monolayer graphene film.
 12. The homoepitaxial tunnel barriertransport device with functionalized graphene-on-graphene of claim 11,wherein the chemically modified monolayer graphene film is a fluorinatedmonolayer graphene film.
 13. The homoepitaxial tunnel barrier transportdevice with functionalized graphene-on-graphene of claim 11, wherein thechemically modified monolayer graphene film is a hydrogenated monolayergraphene film.
 14. The homoepitaxial tunnel barrier transport devicewith functionalized graphene-on-graphene of claim 12, wherein thefluorinated monolayer graphene film is a tunnel barrier.
 15. Thehomoepitaxial tunnel barrier transport device with functionalizedgraphene-on-graphene of claim 11, wherein there is no electricalconnection between the monolayer graphene film and the chemicallymodified monolayer graphene film.
 16. The homoepitaxial tunnel barriertransport device with functionalized graphene-on-graphene of claim 11,wherein the tunneling spin efficiency is from about 26% to about 46%.